The invention relates generally to devices and methods for trapping and removing non-aqueous-phase liquids (NAPLs) from groundwater or sub-surface soil prior to the treatment of the dissolved-phase contaminants by a permeable reactive barrier (PRB).
PRBs are often placed in the path of contaminated groundwater in order to remove dissolved contaminants. PRBs provide a cost-effective means of treating groundwater without mechanical systems.
PRBs are typically configured to remove specific contaminants (i.e., target compounds) as called for on a case-by-case basis. The specific target compound may or may not be a dissolved NAPL. That is to say, the target compound that the PRB intends to remove may be a NAPL at a very low concentration in the groundwater. The following application will discuss the target compound as the contaminant that the PRB is designed to remove, which may or may not include a NAPL. The following discussion will refer to NAPLs as contaminants that the PRB is not configured to process.
In the cases described above, NAPL may be migrating through the subsurface in conjunction with associated dissolved-phase contaminants (i.e., target compounds) in the groundwater. In those cases, the NAPL can cause the PRB to fail in part or in whole, either by 1) NAPL consuming the treatment chemical in the PRB matrix which was intended for low (dissolved) concentrations of target compound or 2) NAPL forming a barrier that is impermeable to groundwater on the up-gradient side of the PRB, or 3) physically coating or in other ways fouling the reactive surfaces of the PRB.
FIG. 1 shows a typical PRB groundwater treatment system 10. The PRB groundwater treatment system is positioned adjacent a water table 14 and includes a low permeability cap 18 and a PRB 22 positioned in the groundwater flow path.
In some instances, hydraulically permeable PRBs 22 are also used to treat groundwater that has been contaminated by hazardous materials such as pesticides, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated dioxins (PCDDs), polychlorinated furans (PCDFs), polychlorinated biphenyls (PCBs), heavy metals, synthetic organic compounds, and the like.
Generally, NAPLs may be broken into two categories; light non-aqueous-phase-liquids (LNAPLs) and dense non-aqueous-phase-liquids (DNAPLs). LNAPLs will tend to the top portion of the ground water flow path while DNAPLs will tend to the bottom portion of the groundwater flow path.
PRBs 22 typically include a permeable reactive layer that has been designed to treat the target contaminants. The reactive material may include, but is not limited to, zero valent iron (ZVI), activated carbon, apatite, organoclay, and/or various type of degradable organic material.
FIG. 1 shows LNAPLs 26 and DNAPLs 30 in a contaminated groundwater flow 34 but not yet in contact with the PRB 22. In the situation depicted in FIG. 1, the PRB 22 is functioning normally and is not yet fouled with NAPLs 26, 30. The contaminated groundwater 34 flows into the PRB 22, the PRB 22 treats a target compound, as desired, and treated groundwater 38 exits the PRB 22.
Referring to FIG. 2, when unexpectedly high concentrations of the target compound and/or NAPLs (e.g., LNAPL 26 or DNAPL 30), prematurely consume the treatment capacity of the PRB 22, the design life of the PRB 22 is decreased. That is, functional failure occurs sooner than planned. The PRB 22 is designed to address dissolved (lower) concentrations of the target compound, rather than higher concentrations of the target compound in addition to or including NAPLs (e.g., LNAPL 26 or DNAPL 30). FIG. 2 shows that concentrations of NAPLs 26, 30 may permeate the PRB 22. Once the PRB 22 is fouled, some groundwater passing through the PRB 22 will be properly treated (i.e., treated groundwater 38), but some groundwater exiting the PRB 22 will be partially treated groundwater 42. That is to say, the partially treated groundwater 42 still contains appreciable levels of undesirable compounds.
Referring to FIG. 3, the DNAPL 30 coats or physically fouls the treatment chemical of the PRB 22, the design life of the PRB 22 is decreased as the treatment chemical is unavailable to react with the target compound(s) in a dissolved state. The design life of the PRB 22 is calculated assuming uniform flow of contaminated groundwater 34 through all parts of the PRB 22. If the flow of contaminated groundwater 34 is focused on only portions of the treatment PRB 22, the full calculated treatment capacity of that PRB 22 is not realized because the focused flow overwhelms those portions of the PRB 22 receiving all the flow and contamination. This increased localized water flow rate can cause premature breakthrough of the treatment media. In addition, a partial blocked PRB 22 will lower the effective transmissivity of the PRB 22 and potentially force a portion of the contaminated groundwater 34 to flow around the PRB 22, resulting in a partial failure (e.g., as shown at the bottom of FIG. 3).
Referring to FIG. 4, the PRB 22 is completely blocked by accumulation of DNAPL 30 and LNAPL 26. The contaminated groundwater 34 bypasses the PRB 22 altogether without any treatment. The contaminants in the water are not treated or removed, essentially rendering the PRB 22 ineffective.
Referring again to FIG. 4, in the case of NAPL 26,30 migrating through the water-permeable PRB 22, the NAPL 26,30 migrates through the PRB 22 without any treatment. The contaminants in the NAPL 26,30 are free to dissolve into the groundwater down-gradient from the PRB 22, essentially rendering the PRB 22 ineffective.